In a turbomachine, such as a steam or gas turbine, a plurality of rotatable arrays of blades or foils are arranged in rows extending radially from an axially aligned rotor. The rows of blades react to the forces of a high pressure fluid flowing axially through the machine to produce rotation of the rotor and the blade rows. During operation the rotating blades experience centrifugal and vibrational forces which generate large stresses on the blade attachment structure and could affect blade integrity. In addition, the efficiency of work performed across each blade row is limited by the amount of fluid which flows between the blade tips and the turbine casing without contributing to blade rotation.
In order to minimize these mechanical and thermodynamic effects, the blade tips in each row are covered by a segmented shroud ring which forms a circumferential sealing surface for limiting stream leakage as well as providing the necessary constraint to control blade vibration and reduce stress levels at the base and root of each blade. To perform these functions the shroud segments should firmly abut one another so as to secure the blades in proper alignment during dynamic loading.
In the past, shroud rings have been attached to the blades by riveting each ring segment to one or more blade tips via tenons which are integrally formed with each blade. In order to strengthen the connection between the shroud and the blading, many blades are formed with integral shroud segments rather than tenons. FIGS. 2, 3 and 4 illustrate an integral shroud segment 11 formed on a typical side entry blade 13. Each shroud segment 11 has a leading planar edge 15 and a trailing planar edge 17 such that when adjoining blades are installed, as illustrated in the partial view of a blade row shown in FIG. 1, the leading shroud edge 15 of each blade abuts the trailing shroud edge 17 of an adjacent blade.
A prior method for installing rows of blade having integral shroud segments has required first wedging the Christmas tree shaped root 19 of an initial blade in registry with a first complementary steeple shaped groove 21 on the turbine rotor 23 to secure the blade portion 24 in radial alignment with the axis of rotation. The rotor 23 is then turned about its axis in order to sequentially install adjacent blades in one circumferential direction along the rotor. Typically, the initial and the subsequent blades have been installed from a fixed point, e.g., 60 degrees from the top-most position about the rotor. According to this method, after installing the initial blade and rotating it beyond the fixed point, a second blade is loosely shimmed in place from the bottom of a second rotor groove 21 adjacent the initial blade. With the leading edge 15 of the shroud segment on the second blade positioned against the trailing edge 17 of the shroud segment on the first blade, this prior method has relied on the gravitational weight of the second blade to create a compact fit between the shroud segments. The sequential installation of blades behind the second blade in this same manner has resulted in a relatively close fit between the shroud segments on additional blades. This installation process continued until the second to the last blade was positioned in its rotor groove. Next, a jack is installed in the one remaining opening in order to compress the shrouds of the installed blades against one another thus expanding the opening to receive a final closing blade in a tight fit to complete the blade row.
Blade installation methods which rely on the weight of a plurality of blades pressing against one another to create a tight fit between shroud segments may cause the initial wedged blade to move out of radial alignment. Although subsequent compression of the shroud segments with a jack prior to inserting the final blade can have the effect of realigning the blades, there is little control over such realignment. Furthermore, the effectiveness of such installation methods for creating a compact blade fit is inversely proportional to blade weight. Therefore, it is desirable to provide an improved installation method which assures proper blade alignment and which provides a sufficiently tight fit for lightweight blades.